The present invention relates to a semiconductor memory device, more particularly a semiconductor memory device, in which the occurrence of a soft error is minimized, and a process for producing the semiconductor memory device.
In semiconductor memory devices, the integration degree of the memory circuit has been enhanced continually over the years, and a dynamic memory cell having a capacity of 64K bits and one transistor per memory cell is mainly employed for a memory circuit at the present time. Since the provision of a memory cell having a minute surface ,area is presently desired, a dynamic memory cell having a surface area smaller than that of the 64K bit dynamic cell will be produced in the future.
Dynamic memory cells must be refreshed, so as to prevent the destruction of the information which is stored in the memory cell as the existence or non-existence of electric charges in the form of minority carriers. The destruction of the information occurs due to generation of thermal carriers in the depletion regions which spread out in the semiconductor substrate of the memory cell. In operating the memory cells, a positive voltage is applied to a semiconductor substrate via a MOS electrode of the memory cells, when the semiconductor substrate has a P type conductivity. As a result of the application of the positive voltage, a depletion layer spreads in the semiconductor substrate according to the number of minority carriers stored in the memory cell. If electron-hole pairs are generated in the semiconductor substrate of the memory cell, during the non-existence state of the minority carriers, namely the storing state of the information "0", holes are swept out from the depletion region, but electrons are diffused toward and stored at the interface between the substrate and an SiO.sub.2 layer under the MOS electrode. As a result, the information which is stored in the substrate surface is destroyed.
Recently, attention has been directed to another reason for the generation of electron-hole pairs, which is the generation of electron-hole pairs caused by absorption of .alpha.-particles by the memory cells. Some of the electrons of the so generated electron-hole pairs stay in the depletion layer and others of the electrons, which are collected in the depletion layer, destroy the information. Such destruction is referred to as a soft error. One source of .alpha.-rays is cosmic rays and another source is emissions from the radioactive materials contained in a package material. As long as the semiconductor device mounted in the package is used in ambient air, the problem of soft error inevitably occurs.